Waste. The actual people running the PIA VPN service don't recommend running VPN and proxy together, as it's a waste and un-needed additionally.

I'm guessing most use the VPN if they have both, like from PIA.

PIA added the proxy(and the ppbt protocol version) for situations where openvpn isn't supported on the platform, so as an alternative.

Personally I prefer a proxy, because little better speed, and program-dependant, as I only need it for torrenting(though I have scripts that do the same anyway with the VPN, but linux only, as it's a little harder in windows to do, or atleast not as simple). Also, running openvpn along-side takes 22% CPU extra of my I3 intel CPU, because of the encryption, which isn't really needed for torrenting, except if beeing throttled(can be forced in torrent client too, but will lower the amount of connectable peers).

Newer versions of utorrent and qbittorrent, with proper settings applied seems to work good, but deluge needs ltconfig plugin and 2 options defined to be safe: force-proxy and anonymous-mode, where the former is the most important and the later isn't that much but still nice. If not using ltconfig, then you're only good until a possible connection-drop occure and then libtorrent failsafes back to non-proxy regular connection instead.

Proxying depends on the app to be bug-free to not leak your ip, so you need to trust or test your app with it. Btw, libtorrent 1.1.1 has added incoming TCP connections supported now through socks5 bind command, but I haven't tested if it works good yet, but pretty cool imho. Before, such was only supported through UDP, so PEX and DHT bound incoming-connections, and uploads through outgoing-connections of course.

If using a VPN, then bind the torrent client to the local VPN IP, to get 100% kill-switch functionality.

If using a proxy, then use https trackers only, so the full url isn't seen by your ISP, only the first part of it, so they can't see release name of torrent, but just that some torrent-site is used. Some have reported getting DMCA letters with newer versions of utorrent, and with proper settings applied(as seen from screenshot), but there ISP was always comcast or verizon, which has spying-software installed from the industry(read an article on it posted by PIA on there page once) and reports too them when infringements are seen, and i'm here guessing the letter was gotten through using a non-https tracker, so just that the torrent meta-file was downloaded i'm guessing triggered it, but I don't know and am guessing here only.

A paid proxy has no worse downtime than a VPN(they use multiple IPs in different countries too, just like VPNs) and you won't get a letter if using a libtorrent-based client with force-proxy enabled or newer utorrent(-server) properly configured too, and using https trackers just in case. You get full IP masking then, which is what's needed here, as encryption is only useful for throttling in our circumstances, and even then, can be forced in client alternatively though with lesser connectable peers.

Almost never. Use a VPN for torrenting. Use a proxy only for unblocking since a proxy will just change your IP so you can access the same app/site from another region (as in, fool or work your way around it.) If unblocking is your only need and there's no real check on you getting around the application or that website, then a proxy would do to and a VPN would be overkill but other than that, i think VPN gets the edge because of encryption. I've mentioned it elsewhere too. check that out too if you like.

The encryption is useful for 1 maybe two things; Avoding traffic shaping from your ISP(or atleast making the chances for avoding it better), and if existing - avoiding any possible snooping of your downloads by your ISP.

When you get a DMCA letter from your ISP, then it's generally described by ISPs as being a third-party company which monitors torrent-swarms and collects IPs with infringements of the media firms they represent and then they contact the ISP and let them forward a DMCA letter to you from them. My point is that the most important thing imho is IP masking, which a proxy also does. There have been some rumors that ISPs also monitor themselves what there users download, but that needs alot of time and effort(DPI) and e.g. much torrent-traffic already is encrypted as first try and only gets un-encrypted if the peer isn't supporting it. Still, also for IP masking a VPN is your safest bet because with a proxy we have to rely on the torrent-client itself not leaking our real IP, as the torrent-spec features several mechanisms for that where bugs could occure and which VPNs aren't a subject off in the same way, though we still need proper measures for kill-switch functionality of course.

Lastly, a VPN additionally often features port-forwarding to increase your connectivity(through additionally accepting incoming-connections), though that aside, the missing encryption-overhead of proxy's can make for little faster speed if the bandwidth was the same(and no port-forwarding enabled), and also lightens your CPU load, as openvpn encryption does get a little taxing on faster speeds.